Instruments exist which measure fluid velocity by frequency based measuring (Doppler), for example by emitting an ultrasonic or microwave carrier signal that echoes off targets such as particulate matter, air bubbles, etc., carried in a flowing liquid and returns with its mean frequency shifted by the Doppler Effect. Common usage for such instruments is in the measurement of open-channel flow, as for instance in a wastewater collection sewer. Instruments of this type estimate fluid velocity from the observed Doppler frequency shift.
The Doppler shift can comprise a shift in a frequency of the reflected signal versus the originally transmitted signal, due to motion of an object toward or away from the Doppler transmitter device. The Doppler shift can be subsequently processed to determine the velocity. Further, the change in frequency of the reflected signal can be used to determine the direction of motion of the object, either toward or away from the Doppler measurement device.
In the measurement of fluid flow, a Doppler shift can be measured using continuous or pulsed waves transmitted through the fluid in order to detect a fluid velocity. Consequently, the waves can be transmitted from within the fluid, including parallel to or at an angle from the fluid surface.
Various methods exist for processing of the returned signal, but most involve some type of spectral analysis. Typically, the normalized power spectral density (PSD) of the returned signal is used as a surrogate for the probability density function (PDF) that describes individual particle velocities. In some instruments, the magnitude spectrum of the returned signal is used instead of the power spectrum. Both the magnitude spectrum and the power spectrum are examples of a velocity spectrum. The velocity spectrum is then used to estimate mean velocity, peak velocity, maximum-likelihood velocity, or some other statistic that is relevant to the flow.
One method of Doppler processing converts frequency values into digital values and uses a Fast Fourier Transform (FFT) to convert the time domain signals into the frequency-domain. The frequency-domain will include a strong Doppler measurement response amplitude in one or more frequency bins, representing the Doppler shift in the reflection from the fluid. The amount of Doppler shift in a reflection depends on the velocity of the target from which it is reflected. Consequently, a large fluid velocity will result in a large shifted distance from the carrier frequency. A velocity spectrum produced by the FFT processing will typically generate two obvious peaks. One peak is associated with the non-Doppler-shifted carrier wave energy. The carrier wave energy is present in the return signal due to some combination of crosstalk and reflection from stationary objects, such as a flow channel boundary or other boundary surface. In an instrument producing a two-sided velocity spectrum, this is the central peak. In an instrument producing a one-sided velocity spectrum, this is typically the left-most peak. The carrier peak will often be the highest peak in the velocity spectrum and will be comparatively narrow. The other peak will represent the measurement reflection obtained from and representative of the fluid flow. The location of this Doppler reflection peak relative to the carrier wave peak will depend on the speed and direction of the fluid flow. A faster fluid flow will be located farther from the carrier wave peak. In instruments producing a two-sided velocity spectrum, the location of the Doppler reflection peak with regard to the carrier wave peak will indicate the flow direction. In such instruments, a peak location to the right of the carrier position will indicate one flow direction, and a peak location to the left of the carrier position will indicate the opposite flow direction. Instruments producing one-sided velocity spectra do not distinguish the flow direction because they allow spectral components from one side of the carrier position to be aliased or mirrored to the other side. These “one-sided” Doppler instruments can measure the velocity magnitude, but not its sign.